1. Field of the Invention
The present invention relates to a magnetic recording medium, a method for producing the same, and a magnetic recording apparatus. In particular, the present invention relates to a magnetic recording medium based on the perpendicular magnetic recording system preferably usable for the high density recording, a method for producing the same, and a magnetic recording apparatus.
2. Description of the Related Art
In recent years, the information oriented society is remarkably advanced, and it becomes possible to process, for example, not only the character information but also the voice information and the image information at high speeds. One of the apparatuses capable of processing the information as described above at the high speed is a magnetic recording apparatus which is installed, for example, to the computer. At present, the development is advanced in relation to the magnetic recording apparatus in order that a further small size is realized while improving the recording density.
In the typical magnetic recording apparatus, a plurality of magnetic disks are rotatably installed on a spindle. Each of the magnetic disks comprises a substrate and a magnetic film (recording layer) which is formed on the substrate. Information is recorded by forming magnetic domains having specified directions of magnetization in the recording layer.
Conventionally, the direction of magnetization, which is adopted for the recording in the recording layer of the magnetic disk, is the in-plane direction of the recording layer. This system is called “in-plane magnetic recording system”. The realization of the high density recording on the magnetic recording medium based on the in-plane magnetic recording system can be achieved by thinning the film thickness of the recording layer, providing fine and minute grain diameters of magnetic crystal grains which constitute the recording layer, and reducing the magnetic interaction between the respective magnetic grains. However, when the magnetic grains are made to be fine and minute and the magnetic interaction between the respective crystal grains is reduced, then a problem has arisen such that the thermal stability of the magnetization of the recorded bits (magnetic marks) is deteriorated.
The perpendicular magnetic recording system has been suggested in order to solve the problem involved in the in-plane magnetic recording system as described above. In this system, a material is used for the recording layer, in which the perpendicular component of the residual magnetization with respect to the film surface is larger than the in-plane component. The direction of magnetization recorded in the recording layer is directed perpendicularly to the substrate. As a result, a magnetostatically stable state is obtained between the adjoining bits, and the recording transition area is sharpened. When a layer (soft magnetic back layer), which is formed of a soft magnetic material, is further added between the recording layer and the substrate, it is possible to steepen the magnetic field applied to the recording layer when information is recorded. As a result, it is possible to record information on the material having high magnetic anisotropy, and it is possible to improve the thermal stability of the magnetization of the bits. Therefore, the recording can be performed at a higher density.
At present, CoPtCr-based alloys are used for the recording layer of the magnetic recording medium based on the in-plane magnetic recording system (such a medium will be hereinafter referred to as “CoPtCr-based alloy medium”). Magnetic recording media, which use the CoPtCr-based alloy for the recording layer, are mainly researched and investigated for the magnetic recording medium based on the perpendicular magnetic recording system as well, in the same manner as in the in-plane magnetic recording system. The CoPtCr-based alloy medium has the following feature. That is, the CoPtCr-based alloy medium forms the two-phase separation structure which is composed of the ferromagnetic crystal grains having a high Co concentration and the non-magnetic crystal grain boundary having a high Cr concentration. The magnetic interaction between the crystal grains is reduced by the non-magnetic crystal grain boundary. Owing to this effect, it is possible to realize the low noise of the medium which is required for the high density recording.
However, in order to respond to the realization of the recording at higher densities, it is required that the magnetic interaction between the crystal grains is further reduced, and the magnetic thermal stability of the bits is simultaneously further enhanced. A method is known for this purpose, in which oxygen is added to the recording layer to oxidize the crystal grain boundary. Such a method is carried out by adding an oxide to a target when the recording layer is formed by means of the sputtering method, or by forming the recording layer in an oxygen gas atmosphere (see, for example, T. Oikawa et al., “Microstructure and Magnetic Properties of CoPtCr—SiO2 Perpendicular Recording Media”, IEEE Trans. Magn., vol. 38, pp. 1976–1978, 2002). In the recording layer formed by the method as described above, a structure, in which the magnetic crystal grains of the recording layer are surrounded by the oxide, i.e., a so-called the granular structure, is formed. The magnetic recording medium, which has the recording layer of the granular structure as described above, is called “oxide-containing CoPtCr-based alloy medium”.
In the case of the oxide-containing CoPtCr-based alloy medium, the crystal grains can be subjected to the separation by the aid of the oxide. Accordingly, it is unnecessary to effect the phase separation by performing the heating as performed for the CoPtCr-based alloy medium containing no oxide. Therefore, the oxide-containing CoPtCr-based alloy medium has such a feature that the crystal grain growth is controlled more easily and minute crystal grains are formed more easily. Further, T. Oikawa et al., “Microstructure and Magnetic Properties of CoPtCr—SiO2 Perpendicular Recording Media”, IEEE Trans. Magn., vol. 38, pp. 1976–1978, 2002 discloses that the S/N ratio (signal-to-noise ratio) is high in the high recording density state, and the thermal stability of the bits can be enhanced, because the magnetic anisotropy of crystals can be improved while reducing the magnetic interaction between the crystal grains in the oxide-containing CoPtCr-based alloy medium as compared with the CoPtCr-based alloy medium containing no oxide.
On the other hand, a magnetic recording medium, which has a recording layer of the granular structure, has been hitherto suggested for the magnetic recording medium based on the in-plane magnetic recording system as well, in order to reduce the medium noise during the high density recording (see, for example, Japanese Patent Application Laid-open No. 2002-208127, pp. 3–5, FIGS. 1 and 2). This patent document discloses that a recording layer, which is formed on an underlayer, is composed of a plurality of magnetic layers containing oxide or the like, and the concentration of the oxide or the like of each of the magnetic layers for constructing the recording layer is changed. In the case of the magnetic recording medium disclosed in Japanese Patent Application Laid-open No. 2002-208127, in order to simultaneously satisfy both of the epitaxial growth of the recording layer and the improvement in the recording and reproduction characteristics, the oxide concentration is increased in the uppermost layer (magnetic layer provided most separately from the underlayer on the side opposite to the underlayer) in the recording layer, and the oxide concentration is decreased in the magnetic layers disposed thereunder. It is intended to realize the low noise by increasing the oxide concentration in the uppermost layer. The oxide concentration is decreased in the magnetic layers disposed thereunder to improve the lattice match between the underlayer and the recording layer thereby so that the epitaxial growth of crystal grains in the recording layer is facilitated. Further, Japanese Patent Application Laid-open No. 2002-208127 discloses that a granular magnetic film, in which Pt and Cr are increased/decreased, is used for the magnetic layer disposed at a lower position in the recording layer in order to facilitate the epitaxial growth of crystal grains in the recording layer.
According to a verifying experiment performed by the present inventors, the following fact has been revealed. That is, the magnetic characteristics are deteriorated when the oxide concentration in the recording layer is further increased to obtain further fine and minute crystal grains in order to realize a higher recording density on an oxide-containing CoPtCr-based alloy medium based on the perpendicular magnetic recording system. Specifically, the magnetization curve was measured in the perpendicular direction (direction of the easy axis of magnetization) with respect to the film surface of the oxide-containing CoPtCr-based alloy medium. As a result, the squareness ratio of the magnetization curve was greatly deteriorated when the oxide concentration in the recording layer was too high. In the case of the magnetic recording medium having a low squareness ratio, magnetic domains (reverse magnetic domains), in which the magnetization is in a direction opposite to the direction of application of the magnetic field in the residual magnetization state, are formed in the recording layer. As a result, when randomly recorded information is reproduced, then the low frequency components of the medium noise are greatly increased, and the bit error rate is deteriorated. Therefore, it is required that the magnetic characteristics of the recording layer are suppressed from the deterioration in order to respond to the demand for the further high density recording on the oxide-containing CoPtCr-based alloy medium based on the perpendicular magnetic recording system.